In the art of invention molding, various control systems and monitoring systems are used in order to achieve more desirably molded article. Injection molding machines are usually of reciprocating screw type by which the material to be molded is plasticized through the application of heat and the mechanical working of the material by the rotation of a screw within a plasticizing chamber. When enough plasticized material has been accumulated, the screw which is joined with a ram of a piston in a hydraulic cylinder advances toward a nozzle orifice by pressure exerted on the piston in the hydraulic cylinder and injects the plasticized material into a mold cavity. It is a fundamental art of injection molding to control injection pressure, injection velocity, viscosity of the plasticized material which has relation to temperature of a heating barrel, back pressure of a screw and rotation speed of a screw during plasticizing, in a predetermined value. Many systems to control these factors have been invented. U.S. Pat. No. 3,642,404 and U.K. Pat. No. 1,296,029 also relate to a control system of an injection molding, of which the invention provides a device which detects a molding filling moment by differentiating an injection pressure value P with a differential time, or the differentiated value dP/dT and a control system which changes an injection pressure for accumulating the plasticized material into a mold cavity to a lower holding pressure than said injection pressure. Concerning monitoring system, a monitoring device in the prior art watches the fluctuation of injection pressure and injection velocity, and temperature of a heating barrel respectively. In monitoring system, it is essential to select some effective monitoring factor in order to mold the good quality and uniformity of the articles. FIG. 1 thereof, an oscillograph of a hydraulic pressure in a hydraulic cylinder chamber is shown. As coldness of a narrow gate portion of mold cavity in early time of injection causes viscosity of melted material to increase, the hydraulic pressure in the hydraulic cylinder chamber increases. An then the narrow gate portion being heated soon, the hydraulic pressure decreases slightly. Consequently a peak pressure P1 in the hydraulic cylinder chamber occurres at the time T1 as shown in FIG. 1. When melted material is filled in the mold cavity, the hydraulic pressure is changed to a holding pressure, which is kept to a lower pressure during predetermined time than the pressure during injection. Significance of the peak pressure is very important, because a change of hydraulic pressure in the hydraulic cylinder, a injection speed, a viscocity of melted material has effect on a time from a moment of injection start to a occurrence of peak pressure and the pressure at the moment of occurrence.
It is an object of the present invention to provide effective factors for a monitoring system.
Another object of this invention is to provide an improved injection apparatus having a monitoring system detecting a peak pressure in the hydraulic cylinder and measuring a time from a moment of injection start to a moment of occurrence of said peak pressure.